refactor(units): route all conversions through UnitPolicy.convert

Delete the legacy _convertUnitValue helper on the domain and the
duplicate convertUnitValue export on curveNormalizer; both were
identical to UnitPolicy.convert. Callers in flowController, the
curve normalizer, and buildQHCurve now go through this.unitPolicy.
The contract in .claude/refactor/CONTRACTS.md §6 named these as the
target migration; this finishes the rollout for rotatingMachine.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/flow/flowController.js

@@ -50,7 +50,7 @@ class FlowController {
           return await host.executeSequence(parameter);
 
         case 'flowmovement': {
-          const canonicalFlowSetpoint = host._convertUnitValue(
+          const canonicalFlowSetpoint = host.unitPolicy.convert(
             parameter,
             host.unitPolicy.output.flow,
             host.unitPolicy.canonical.flow,